The day after Republican presidential candidate Mitt Romney released his tax returns for 2011, the campaign to re-elect President Barack Obama released an ad questioning why more years weren't made public.

"This is only the tip of the iceberg — Romney has only released two years of tax returns, both from after he started running for President," the ad's voice-over states.

"He's still failing to match the precedent set by other Presidential candidates," it adds, highlighting how Romney's father, George, and President Obama both released 12 years of tax returns.

George H.W. Bush made the greatest release of the recent presidents, making public 19 years of tax returns, the ad points out.

While Romney focused on the fact he gave 30 percent of his 2011 earnings to charity and paid more taxes than required under law, the ad highlights what it calls "troubling new information."

This information includes Romney investments in a Chinese oil firm and stocks in Russia's state-owned energy company, Gazprom, as well as dozens of foreign accounts. He also invested in multiple foreign currencies including the Norwegian krone, the Australian dollar, the Swedish krona and the Canadian dollar.

"Why won't Mitt Romney match other Presidential candidates, including his father? Why won't he come clean with the American people? What else is he hiding?" the ad concludes.

Senator John McCain, writing to Romney supporters by email Friday, preemptively criticized the president for focusing on the tax return release and not "the issues that voters care about."

"While President Obama and Democrats will try to distract voters, Mitt Romney and Paul Ryan are focused on fixing the economy, getting Americans back to work and ensuring a better future for our children and grandchildren," McCain added.